The present invention relates to integrated circuits and more particularly, to improving the compromise between noise performance and power consumption of a custom integrated circuit.
In applications, such as some X-ray imaging applications, that require low level, low noise analog signal processing, often a field effect transistor (FET), or a device with a FET, is chosen as the input device. FET devices have desirable high impedance, low noise characteristics. It is known that the noise of an FET device, when properly biased will be inversely proportional to the 4th root of the magnitude of the channel (source to drain) bias current. One means of improving the noise performance of the circuit then, is to increase the bias current through the input FET.
Increasing the bias current increases power dissipation, in direct proportion to the bias current, which is an undesirable side effect. This is further compounded when a large number of independent channels are required at the system level, as the power dissipation for the system now increases proportionally by the number of channels required. Yet this increased power dissipation is wasted when the circuit is used in conjunction with higher noise input signals.
It would be desirable, then, to have a means for selecting low noise performance or low power dissipation in the analog front end of a custom integrated circuit. For economic reasons, it is desirable to apply the custom integrated circuit which implements the analog signal processing to as many applications as possible, whereby one design solves multiple problems, rather than requiring a unique solution for each problem.
X-ray applications may be diagnostic, requiring high input signal levels and, therefore, requiring less circuit noise control; or fluoroscopic, having low input signal noise and, therefore, requiring improved circuit noise control. When the signal level can be determined prior to analog processing, the present invention provides for a means to select between low power or low noise for the circuit.
In accordance with one aspect of the present invention, additional current sources which improve the noise performance of the circuit are switched into the circuit when better noise performance is required and switched out of the circuit in order to conserve power when poorer noise performance can be tolerated.
Accordingly, it is an object of the present invention to provide a means to select between low noise performance or low power dissipation for a custom integrated circuit. This has the advantage of conserving power and reducing heat for the circuit, according to actual user requirements.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.